<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  
  <meta http-equiv="X-UA-Compatible" content="IE=edge" >
  <title>正确理解JS中的变量赋值 | 默默地向前端靠拢</title>
  <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
  <meta name="description" content="JS中的变量赋值过程和原理，透彻理解JS的变量赋值与原理">
<meta name="keywords" content="NodeJs的异步IO">
<meta property="og:type" content="article">
<meta property="og:title" content="正确理解JS中的变量赋值">
<meta property="og:url" content="http://yoursite.com/2017/06/14/variable/index.html">
<meta property="og:site_name" content="默默地向前端靠拢">
<meta property="og:description" content="JS中的变量赋值过程和原理，透彻理解JS的变量赋值与原理">
<meta property="og:locale" content="zh-Hans">
<meta property="og:updated_time" content="2017-06-14T03:54:25.000Z">
<meta name="twitter:card" content="summary">
<meta name="twitter:title" content="正确理解JS中的变量赋值">
<meta name="twitter:description" content="JS中的变量赋值过程和原理，透彻理解JS的变量赋值与原理">
  
    <link rel="alternative" href="/atom.xml" title="默默地向前端靠拢" type="application/atom+xml">
  
  
    <link rel="icon" href="/img/head.jpg">
  
  <link rel="stylesheet" href="/css/style.css">
</head>

<body>
  <div id="container">
    <div class="left-col">
    <div class="overlay"></div>
<div class="intrude-less">
	<header id="header" class="inner">
		<a href="/" class="profilepic">
			
			<img lazy-src="/img/head.jpg" class="js-avatar">
			
		</a>

		<hgroup>
		  <h1 class="header-author"><a href="/">心情</a></h1>
		</hgroup>

		
		<p class="header-subtitle">你所浪费的今天，是昨天死去的人奢望的明天；你所厌恶的现在，是未来的你回不去的曾经。</p>
		

		
			<div class="switch-btn">
				<div class="icon">
					<div class="icon-ctn">
						<div class="icon-wrap icon-house" data-idx="0">
							<div class="birdhouse"></div>
							<div class="birdhouse_holes"></div>
						</div>
						<div class="icon-wrap icon-ribbon hide" data-idx="1">
							<div class="ribbon"></div>
						</div>
						
						<div class="icon-wrap icon-link hide" data-idx="2">
							<div class="loopback_l"></div>
							<div class="loopback_r"></div>
						</div>
						
						
						<div class="icon-wrap icon-me hide" data-idx="3">
							<div class="user"></div>
							<div class="shoulder"></div>
						</div>
						
					</div>
					
				</div>
				<div class="tips-box hide">
					<div class="tips-arrow"></div>
					<ul class="tips-inner">
						<li>菜单</li>
						<li>标签</li>
						
						<li>友情链接</li>
						
						
						<li>关于我</li>
						
					</ul>
				</div>
			</div>
		

		<div class="switch-area">
			<div class="switch-wrap">
				<section class="switch-part switch-part1">
					<nav class="header-menu">
						<ul>
						
							<li><a href="/">主页</a></li>
				        
							<li><a href="/archives">所有文章</a></li>
				        
							<li><a href="/categories/随笔">随笔总结</a></li>
				        
							<li><a href="/categories/笔记">学习笔记</a></li>
				        
						</ul>
					</nav>
					<nav class="header-nav">
						<div class="social">
							
								<a class="github" target="_blank" href="http://springxiege.gitbub.io" title="github">github</a>
					        
								<a class="weibo" target="_blank" href="http://weibo.com/springxiegel" title="weibo">weibo</a>
					        
								<a class="mail" target="_blank" href="mailto://springxiege@sina.cn" title="mail">mail</a>
					        
						</div>
					</nav>
				</section>
				
				
				<section class="switch-part switch-part2">
					<div class="widget tagcloud" id="js-tagcloud">
						<a href="/tags/Git/" style="font-size: 10px;">Git</a> <a href="/tags/HTML5/" style="font-size: 10px;">HTML5</a> <a href="/tags/JS/" style="font-size: 20px;">JS</a> <a href="/tags/JavaScript/" style="font-size: 13.33px;">JavaScript</a> <a href="/tags/Node/" style="font-size: 10px;">Node</a> <a href="/tags/NodeJS/" style="font-size: 16.67px;">NodeJS</a> <a href="/tags/NodeJs/" style="font-size: 10px;">NodeJs</a> <a href="/tags/React-js/" style="font-size: 13.33px;">React.js</a> <a href="/tags/hexo/" style="font-size: 10px;">hexo</a>
					</div>
				</section>
				
				
				
				<section class="switch-part switch-part3">
					<div id="js-friends">
					
			          <a target="_blank" class="main-nav-link switch-friends-link" href="http://www.52cik.com/">乱码大大的博客</a>
			        
			          <a target="_blank" class="main-nav-link switch-friends-link" href="http://webcaiqingchong.github.io/">菜青虫的博客</a>
			        
			        </div>
				</section>
				

				
				
				<section class="switch-part switch-part4">
				
					<div id="js-aboutme">我是心情，我是一名前端工作者，目前靠Coding讨生活！平时喜欢玩三国杀，听音乐，运动也喜欢，只是现在运动项目做得少了。</div>
				</section>
				
			</div>
		</div>
	</header>				
</div>

    </div>
    <div class="mid-col">
      <nav id="mobile-nav">
  	<div class="overlay">
  		<div class="slider-trigger"></div>
  		<h1 class="header-author js-mobile-header hide">心情</h1>
  	</div>
	<div class="intrude-less">
		<header id="header" class="inner">
			<div class="profilepic">
			
				<img lazy-src="/img/head.jpg" class="js-avatar">
			
			</div>
			<hgroup>
			  <h1 class="header-author">心情</h1>
			</hgroup>
			
			<p class="header-subtitle">你所浪费的今天，是昨天死去的人奢望的明天；你所厌恶的现在，是未来的你回不去的曾经。</p>
			
			<nav class="header-menu">
				<ul>
				
					<li><a href="/">主页</a></li>
		        
					<li><a href="/archives">所有文章</a></li>
		        
					<li><a href="/categories/随笔">随笔总结</a></li>
		        
					<li><a href="/categories/笔记">学习笔记</a></li>
		        
		        <div class="clearfix"></div>
				</ul>
			</nav>
			<nav class="header-nav">
				<div class="social">
					
						<a class="github" target="_blank" href="http://springxiege.gitbub.io" title="github">github</a>
			        
						<a class="weibo" target="_blank" href="http://weibo.com/springxiegel" title="weibo">weibo</a>
			        
						<a class="mail" target="_blank" href="mailto://springxiege@sina.cn" title="mail">mail</a>
			        
				</div>
			</nav>
		</header>				
	</div>
</nav>

      <div class="body-wrap"><article id="post-variable" class="article article-type-post" itemscope itemprop="blogPost">
  
    <div class="article-meta">
      <a href="/2017/06/14/variable/" class="article-date">
  	<time datetime="2017-06-14T03:51:14.000Z" itemprop="datePublished">2017-06-14</time>
</a>
    </div>
  
  <div class="article-inner">
    
      <input type="hidden" class="isFancy" />
    
    
      <header class="article-header">
        
  
    <h1 class="article-title" itemprop="name">
      正确理解JS中的变量赋值
    </h1>
  

      </header>
      
      <div class="article-info article-info-post">
        
	<div class="article-tag tagcloud">
		<ul class="article-tag-list"><li class="article-tag-list-item"><a class="article-tag-list-link" href="/tags/JS/">JS</a></li><li class="article-tag-list-item"><a class="article-tag-list-link" href="/tags/JavaScript/">JavaScript</a></li></ul>
	</div>

        
	<div class="article-category tagcloud">
	<a class="article-category-link" href="/categories/随笔/">随笔</a>
	</div>


        <div class="clearfix"></div>
      </div>
      
    
    <div class="article-entry" itemprop="articleBody">
      
        <h1 id="正确理解JS中的变量赋值"><a href="#正确理解JS中的变量赋值" class="headerlink" title="正确理解JS中的变量赋值"></a>正确理解JS中的变量赋值</h1><p>简单理解就是数字和字符串是复制，对象和数组是地址引用。但如果是引用，如果你对参数进行重新赋值的话，会改变参数引用的地址。函数外部的数组或者对象就不会被修改。<br><a id="more"></a></p>
<p>在看到上面这句话之前，本来我是对变量赋值的概念还不是很理解的，因为最开始我就直接认为变量赋值就是字面上的复制;然而在最近的一次新项目开发中，遇到了一个变量赋值的问题，让我对变量赋值的概念有了不一样的理解，我就又直接的认为变量赋值就是地址引用。但是，这特么也是错误的理解。虽然之前对这样的理解充满着疑惑，也没有去查阅资料去解决心中的问题，就这样让这个知识点从我脑中遗失了。</p>
<p>由于最近几天想脑补一下JS的基础知识，提升自己的基础知识水平，所以又重新开始翻阅各种资料追根朔源。今天在群里的某大神帮助下，基本上算是比较透彻的理解了变量赋值。简单的一句话，说出了变量赋值的原理：</p>
<blockquote>
<p>简单理解就是数字和字符串是复制，对象和数组是地址引用。但如果是引用，如果你对参数进行重新赋值的话，会改变参数引用的地址。函数外部的数组或者对象就不会被修改。</p>
</blockquote>
<p>下面写几个例子，验证一下JS的变量赋值到底是怎样的。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">var</span> a = <span class="number">3</span>;</div><div class="line"><span class="keyword">var</span> b = a;</div><div class="line"></div><div class="line">b++;</div><div class="line"></div><div class="line"><span class="built_in">console</span>.log(a);  <span class="comment">// return 3;</span></div><div class="line"><span class="built_in">console</span>.log(b);  <span class="comment">// return 4;</span></div><div class="line"></div><div class="line"><span class="keyword">var</span> c = <span class="string">'string'</span>;</div><div class="line"><span class="keyword">var</span> d = c;</div><div class="line"></div><div class="line">c += <span class="string">' is String'</span>;</div><div class="line"></div><div class="line"><span class="built_in">console</span>.log(c);  <span class="comment">// return 'string is String';</span></div><div class="line"><span class="built_in">console</span>.log(d);  <span class="comment">// return 'string';</span></div></pre></td></tr></table></figure>
<p>此时，从上面的例子可以看出，虽然b的值加1，但是a的值没有改变，另外，c变量有一个字符串拼接过程，但d的值没有变化。从而说明数字和字符串类型的变量赋值就是复制过程。</p>
<p>下面再验证一下对象Object和数组Array变量的赋值</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">var</span> arr = [<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>];</div><div class="line"><span class="keyword">var</span> brr = arr;</div><div class="line"></div><div class="line">brr.push(<span class="number">4</span>);</div><div class="line"></div><div class="line"><span class="built_in">console</span>.log(arr);  <span class="comment">// return [1,2,3,4]</span></div><div class="line"><span class="built_in">console</span>.log(brr);  <span class="comment">// return [1,2,3,4]</span></div><div class="line"></div><div class="line"><span class="keyword">var</span> obj = &#123;<span class="attr">a</span>: <span class="number">1</span>, <span class="attr">b</span>: <span class="number">2</span>&#125;;</div><div class="line"><span class="keyword">var</span> obj2 = obj;</div><div class="line"></div><div class="line">obj.c = <span class="number">3</span>;</div><div class="line"></div><div class="line"><span class="built_in">console</span>.log(obj);  <span class="comment">// return &#123;a: 1, b: 2, c: 3&#125;;</span></div><div class="line"><span class="built_in">console</span>.log(obj2);  <span class="comment">// return &#123;a: 1, b: 2, c: 3&#125;;</span></div></pre></td></tr></table></figure>
<p>此时，从上面的例子可以得出一个结论，那就是对象Object类型与数组Array类型的变量赋值，是一个地址引用的过程。</p>
<p>接下来就是验证一下对参数进行重新赋值，是否会改变参数引用的地址，而函数外部的数组或者对象就不会被修改。</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">var</span> arr = [<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>];</div><div class="line"></div><div class="line"><span class="comment">// function change(a) &#123;</span></div><div class="line"><span class="comment">//	a.push(4);</span></div><div class="line"><span class="comment">//	return a;</span></div><div class="line"><span class="comment">//&#125;</span></div><div class="line"></div><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">set</span>(<span class="params">a</span>) </span>&#123;</div><div class="line">	a = [<span class="number">3</span>,<span class="number">2</span>,<span class="number">1</span>];</div><div class="line">	<span class="keyword">return</span> a;</div><div class="line">&#125;</div><div class="line"></div><div class="line"><span class="comment">//var brr = change(arr);</span></div><div class="line"></div><div class="line"><span class="keyword">var</span> crr = set(arr);</div><div class="line"></div><div class="line"><span class="built_in">console</span>.log(arr);  <span class="comment">// return [1,2,3]</span></div><div class="line"><span class="comment">//console.log(brr);  // return [1,2,3,4]</span></div><div class="line"><span class="built_in">console</span>.log(crr);  <span class="comment">// return [3,2,1]</span></div></pre></td></tr></table></figure>
<p>从上面两个例子的结果可以看出，数组Array类型的变量赋值的确是地址的引用，然后如果作为一个函数的参数被引用后被重新赋值了，那么函数外部的数组或者对象就不会被修改了，因而参数引用的地址发生了改变。如果引用的参数在函数内并不是赋值，而是修改参数里的某个属性的话，则参数引用地址不会发生变化，外部的变量属性值也会随之发生变化。</p>
<p>接下来的对象Object类型的变量赋值跟数组Array类型一样，你可以亲手去验证一下，这里不再举例了。</p>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>对于变量的理解，一定要理解透彻，不然在平时写的代码中会出现意想不到的结果，从而让自己陷入困境。在真正理解了变量赋值的过程和原理后，才能在自己的代码中运用自如，规避各种坑。<br>用简单的一句总结变量赋值的过程和原理：数字和字符串类型的变量赋值即复制过程，对象Object类型和数组Array类型的变量赋值即地址引用。但如果是引用，如果你对参数进行重新赋值的话，会改变参数引用的地址。函数外部的数组或者对象就不会被修改。</p>

      
    </div>
    
  </div>
  
    
<nav id="article-nav">
  
    <a href="/2017/06/22/react-summary/" id="article-nav-newer" class="article-nav-link-wrap">
      <strong class="article-nav-caption"><</strong>
      <div class="article-nav-title">
        
          使用Dva手脚架搭建React项目
        
      </div>
    </a>
  
  
    <a href="/2017/02/12/function-arguments/" id="article-nav-older" class="article-nav-link-wrap">
      <div class="article-nav-title">函数参数详解</div>
      <strong class="article-nav-caption">></strong>
    </a>
  
</nav>

  
</article>


<div class="share share_jia">
	<!-- JiaThis Button BEGIN -->
	<div class="jiathis_style">
		<span class="jiathis_txt">分享到: &nbsp; </span>
		<a class="jiathis_button_facebook"></a> 
    <a class="jiathis_button_twitter"></a>
    <a class="jiathis_button_plus"></a> 
    <a class="jiathis_button_tsina"></a>
		<a class="jiathis_button_cqq"></a>
		<a class="jiathis_button_douban"></a>
		<a class="jiathis_button_weixin"></a>
		<a class="jiathis_button_tumblr"></a>
    <a href="http://www.jiathis.com/share" class="jiathis jiathis_txt jtico jtico_jiathis" target="_blank"></a>
	</div>
	<script type="text/javascript" src="http://v3.jiathis.com/code/jia.js?uid=1405949716054953" charset="utf-8"></script>
	<!-- JiaThis Button END -->
</div>






<div class="duoshuo">
	<!-- 多说评论框 start -->
	<div class="ds-thread" data-thread-key="variable" data-title="正确理解JS中的变量赋值" data-url="http://yoursite.com/2017/06/14/variable/"></div>
	<!-- 多说评论框 end -->
	<!-- 多说公共JS代码 start (一个网页只需插入一次) -->
	<script type="text/javascript">
	var duoshuoQuery = {short_name:"true"};
	(function() {
		var ds = document.createElement('script');
		ds.type = 'text/javascript';ds.async = true;
		ds.src = (document.location.protocol == 'https:' ? 'https:' : 'http:') + '//static.duoshuo.com/embed.js';
		ds.charset = 'UTF-8';
		(document.getElementsByTagName('head')[0] 
		 || document.getElementsByTagName('body')[0]).appendChild(ds);
	})();
	</script>
	<!-- 多说公共JS代码 end -->
</div>




</div>
      <footer id="footer">
  <div class="outer">
    <div id="footer-info">
    	<div class="footer-left">
    		&copy; 2017 心情
    	</div>
      	<div class="footer-right">
      		<a href="http://hexo.io/" target="_blank">Hexo</a>  Theme <a href="https://github.com/litten/hexo-theme-yilia" target="_blank">Yilia</a> by Litten
      	</div>
    </div>
  </div>
</footer>
    </div>
    
  <link rel="stylesheet" href="/fancybox/jquery.fancybox.css">


<script>
	var yiliaConfig = {
		fancybox: true,
		mathjax: true,
		animate: true,
		isHome: false,
		isPost: true,
		isArchive: false,
		isTag: false,
		isCategory: false,
		open_in_new: false
	}
</script>
<script src="http://7.url.cn/edu/jslib/comb/require-2.1.6,jquery-1.9.1.min.js"></script>
<script src="/js/main.js"></script>






<script type="text/x-mathjax-config">
MathJax.Hub.Config({
    tex2jax: {
        inlineMath: [ ['$','$'], ["\\(","\\)"]  ],
        processEscapes: true,
        skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
    }
});

MathJax.Hub.Queue(function() {
    var all = MathJax.Hub.getAllJax(), i;
    for(i=0; i < all.length; i += 1) {
        all[i].SourceElement().parentNode.className += ' has-jax';                 
    }       
});
</script>

<script type="text/javascript" src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML">
</script>


  </div>
</body>
</html>